A car bomb struck a U.S. military patrol Saturday in the Iraqi capital, killing two U.S. soldiers and injuring two others, and a Marine died in action in a restive central province, the military said.

Earlier military officials said they had discovered a 600-foot tunnel leading out of the main prison facility for detainees in Iraq. No one had escaped, said an Army spokeswoman, Maj. Flora Lee. She did not know when guards discovered the tunnel.

Camp Bucca (search) holds 6,049 detainees, nearly two-thirds of all those in Iraq, Lee said late Friday. Situated near the southern city of Umm Qasr (search), it is one of three detainee facilities in Iraq.

A bucket cut from a water container and a shovel made of tent material were used to dig the tunnel, Lee said. The opening was under a floorboard of the compound and was concealed with dirt.

Authorities in charge of the compound realized a tunnel was under way after they found dirt in latrines and other places, Lee said.

"There have been a few other attempts at digging a tunnel but nothing of this size," she said.

U.S. guards fired on prisoners during a riot at Camp Bucca on Jan. 31, killing four detainees and injuring six others.

The names of the U.S. troops slain Saturday were withheld pending notification of the family, and no other details were given on the car bombing.

The military also announced Saturday that a Marine was killed a day earlier during a "security and stability" operation in strife-torn Anbar province, a heavily Sunni Arab region west of Baghdad that contains the flashpoint cities of Fallujah and Ramadi.

No further details were given of the death of the Marine, assigned to the 1st Marine Expeditionary Force.

As of Friday, at least 1,524 members of the U.S. military have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.

On Iraq's political front, the interim government's deputy prime minister, Barham Saleh, said Friday that negotiators are intensifying efforts to bring in the country's Sunni Arabs, believed to form the core of Iraq's rebellion. That has caused delays, leading to public frustration with the nascent political process.

"It is not acceptable that two months on from the elections, that Iraq does not have a transitional government yet," Saleh told The Associated Press in an interview. "We are under pressure, and we have to respond to public sentiment and have a government established as soon as possible."

Jawad al-Maliki, a negotiator from the Shiite-led United Iraqi Alliance, said Shiite and Kurdish officials agreed to hold their second National Assembly session on Tuesday.

Since holding their first session on March 16, lawmakers elected in Jan. 30 elections have repeatedly postponed a second meeting because of ongoing negotiations over the make up of the government.

Insurgents trying to undermine the formation of a new government, meanwhile, seemed to intensify their attacks, carrying out four suicide car bombings across Iraq that killed 17 Iraqi security officials and two civilians. Militants have stepped up attacks against Iraqi police and soldiers who are key to an eventual U.S. withdrawal.

Twin suicide car bombings Friday in Iskandriyah, 30 miles south of Baghdad, targeted an Iraqi army convoy and police barracks and killed four policemen, two civilians and an Iraqi soldier, police officials said. Eight other members of the security forces and 15 civilians were injured.

Another suicide car bombing Friday targeted an Iraqi convoy south of Baghdad and left one Iraqi soldier dead and four others injured, police Capt. Muthama Abdul Rida said.

Late Thursday at a checkpoint in the central city of Ramadi, a white sedan was blown up, killing 11 Iraqi soldiers and injuring 14 people — including two U.S. Army soldiers, nine Iraqi security forces, and three civilians — the U.S. military said. The Islamic Army in Iraq posted a statement on the Ansar Web site claiming responsibility.

Another car bomb exploded Friday in the city's center. It targeted a U.S.-Iraqi convoy, but only killed the two attackers in the car.

In Baghdad on Friday, unknown gunmen killed Col. Salman Muhammad Hassan and injured two of his sons as they left a relative's funeral in Baghdad, security officials said. Police also said Friday they found two decapitated bodies clad in Iraqi army uniforms a day earlier on a road north of Baghdad.